SPX down 0.59% but on positive breadth?

Yesterday (8/19) was a strange day indeed. The S&P 500 (SPX) dropped -0.59%, but breadth was positive with over 350 stocks up on the day. This was a day when the average stock beat the biggest stocks, and the RSP (S&P 500 equal weight ETF) rallied 0.47%. Our data show that the SPX has dropped 0.50% or more with at least 350 stocks advancing on the day just nine times. There are no observations prior to mid 2019, and seven of the nine have happened over the last 15 months, marking this as a more recent phenomenon. When the SPX is down 0.05% or more with at least 350 stocks up on the day, forward returns for the index after this signal can be more volatile. See the return profile from Optuma's Signal Tester below.

Source: Optuma